First-Principles Equation of State Calculations of Warm Dense Nitrogen

TL;DR

This study provides a comprehensive first-principles equation of state for nitrogen across various states, revealing detailed plasma behavior and ionization processes in warm dense matter, with results aligning well with experimental data.

Contribution

It extends the nitrogen phase diagram into the dissociation regime using PIMC and DFT-MD, offering new Hugoniot curves and insights into plasma structure and ionization.

Findings

Hugoniot curves match experimental data and previous studies

Identification of two compression maxima related to shell ionization

Observation of evolving plasma structure and ionization with temperature and pressure

Abstract

Using path integral Monte Carlo (PIMC) and density functional molecular dynamics (DFT-MD) simulation methods, we compute a coherent equation of state (EOS) of nitrogen that spans the liquid, warm dense matter (WDM), and plasma regimes. Simulations cover a wide range of density-temperature space, $1.5-13.9$~g$~$cm$^{-3}$ and $10^3-10^9$~K. In the molecular dissociation regime, we extend the pressure-temperature phase diagram beyond previous studies, providing dissociation and Hugoniot curves in good agreement with experiments and previous DFT-MD work. Analysis of pair-correlation functions and the electronic density of states in the WDM regime reveals an evolving plasma structure and ionization process that is driven by temperature and pressure. Our Hugoniot curves display a sharp change in slope in the dissociation regime and feature two compression maxima as the K and L shells are ionized in the WDM regime, which have some significant differences from the predictions of plasma models.